This invention relates to a transmission device for use in switching from an active section to a backup section in a digital communication system. It is to be noted that the transmission device may be supplied either with a plurality of multiplexed signals or with a plurality of nonmultiplexed signals, although each of the multiplexed and the nonmultiplexed signals will be collectively called an input signal.
A conventional transmission device of the type described often comprises not only an active section but also a backup section so as to improve reliability of the transmission device.
For example, such a transmission device may be located between a plurality of high speed transmission lines and a plurality of low speed transmission lines. In this event, the high speed transmission lines are divided into a single backup high speed transmission line and active high speed transmission lines, N in number, while the low speed transmission lines are divided into a single active low speed transmission line and a corresponding low speed backup line. Thus, this structure has a high speed side of a l:N scheme and a low speed side of a 1:1 scheme or a one-plus-one (1+1) scheme.
Alternatively, another conventional transmission device may comprise a duplex structure which is composed of an active section and a backup section. Both the active and the backup sections of the duplex structure may be located between a plurality of input external transmission lines and a plurality of output external transmission lines connected to any other external devices or may be located within an exchange or the like to connect internal input lines to internal output lines.
At any rate, both the active and the backup sections are similar in structure to each other in each of the conventional transmission devices. More specifically, each of the active and the active and the backup sections comprises a signal distributor package and a signal switching package both of which are connected to each other through electric wirings laid on a back board and which may be referred to as first and second packages, respectively. Each signal distributor package comprises a fault detection circuit connected to a selected one of input lines and a signal distributor connected to a plurality of the signal switching packages arranged in the active and the backup sections. This shows that each of the signal distributor packages is crosswise connected to the following signal switching packages on the back board outside of the signal distributor packages and the signal switching packages.
In this structure, each of the signal distributor packages is given an input signal to distribute the input signal to a plurality of distributor output terminals and to produce a plurality of internal output signals in parallel through the distributor output terminals. The internal output signals are delivered to each of the signal switching packages from different ones of the signal distributors. Each of the signal switching packages comprises a switch circuit which selects one of the internal output signals to produce the selected internal output signal as an external output signal. Thus, a plurality of the internal output signals are delivered to each switch circuit of the signal switching packages.
Accordingly, a plurality of input terminals should be provided in each of the signal switching packages so as to receive the plurality of the internal output signals from the plurality of the signal distributor packages.
Herein, it is to be noted that fault detection should be made at every one of the internal output signals given to each switch circuit. This means that a plurality of fault detectors should be included in each of the signal switching packages to detect a fault in each of the internal output signals and should be monitored by a control circuit which carries out switch control operation of the switch circuits included in the signal switching packages.
Under the circumstances, if a fault takes place in the active section, the switch circuits are operated in the plurality of the signal switching packages under control of the control circuit to switch from the active section to the backup section. Thereafter, communication is carried out through the backup section which recovers the fault in the active section. From this fact, it is readily understood that the control circuit should monitor not only the fault detection circuits included in the signal distributor packages but also the fault detection circuits included in the signal switching packages.
In the above-mentioned transmission device, complexed wirings are required on the back board to connect each of the signal distributor packages to the following signal switching packages through the input terminals of the signal switching packages. This makes the wirings difficult on the back board. In addition, a large number of the fault detection circuits should be prepared in the signal distributor packages and the signal switching packages. This makes the distributor packages and the control circuit intricate in structure. In addition, the control circuit should execute an intricate control operation.